Backport of stm32f7 add DTCM to heap and use it on F7 (#6865)

* Backport:stm32f7: stm32_allocateheap: allow use DTCM memory for heap

   Back port of upstrem contrib by Jussi Kivilinna <jussi.kivilinna@haltian.com>

   stm32f7: stm32_allocateheap: allow use DTCM memory for heap

   STM32F7 has up to 128KiB of DTCM memory that is currently left unused.

   This patch adds DTCM to main heap if CONFIG_STM32F7_DTCMEXCLUDE is not enabled.

* px4fmu-v5_default:Enable inclusion of the DTCM in the heap

  CONFIG_MM_REGIONS=3 adds the DTCM region to the heap.